Structural basis for the catalytic mechanism of mammalian 25-kDa thiamine triphosphatase

Abstract

Mammalian soluble thiamine triphosphatase (ThTPase) is a 25-kDa cytosolic enzyme that specifically catalyzes the conversion of thiamine triphosphate (ThTP) to thiamine diphosphate and has an absolute requirement for divalent cations. We have investigated the kinetic properties of recombinant mouse thiamine triphosphatase (mThTPase) and determined its solution structure by NMR spectroscopy. Residues responsible for binding Mg2+ and ThTP were determined from NMR titration experiments. The binding of Mg2+ induced only a minor local conformational change, whereas ThTP binding was found to cause a more global conformational change. We derived a structural model for the mThTPase·ThTP·Mg2+ ternary complex and concluded from this that whereas free mThTPase has an open cleft fold, the enzyme in the ternary complex adopts a tunnel fold. Our results provide a functional rationale for a number of conserved residues and suggest an essential role for Mg 2+ in catalysis. We propose a mechanism underlying the high substrate specificity of mThTPase and discuss the possible role of water molecules in enzymatic catalysis. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.